Hydraulic fracturing is a widely used method for stimulating oil and/or gas production. In performing a fracturing operation, a hydraulic fluid, usually a liquid hydrocarbon, is pumped into the well bore at sufficiently high pressure to fracture the surrounding rock formation to open cracks in the formation through which oil and/or gas can flow into the well bore. Since the cracks which are opened by the fracturing fluid tend to close once the pressure on the fluid is released, it is customary to inject into the well along with the fracturing fluid a suitable particulate proppant material such as sand. The small proppant particles flow into the fractures created in the formation along with the fracturing fluid and serve to prop the fracture open after the fluid pressure is released.
Proppant materials may be either lighter or heavier than the hydraulic fracturing fluid and thus may tend to float or settle out of the fluid prematurely, or otherwise be unevenly distributed in the fracturing fluid. To overcome this problem, it is customary practice to use gelled fracturing fluids which will hold the proppant material in suspension as the fluid flows down the well bore and out into the formation fractures This requires that the gelled fracturing fluid be of sufficient viscosity to hold the proppant material suspended in a slurry or matrix. At the same time, the fluid must not be so viscous that it cannot be readily pumped into the well bore and out into the formation fractures.
Various materials are known which, when admixed with hydrocarbons, will create hydrocarbon gels of various viscosities. However, many of these materials are not suitable for use as hydrocarbon fracturing fluids because of the particular requirements imposed by the environment in which they are used. The gels must be formed at or near the wellhead at ambient temperature Generally, several thousand gallons of normally liquid hydrocarbon such as crude or refined oil, a gelling agent and an activator are blended to form a gel. This mixture then is stored in frac tanks until used.
An ideal gelling agent for forming a gelled hydrocarbon fracturing fluid would be one which, when mixed with the normally liquid hydrocarbon and an activator, forms a gel of sufficient viscosity to hold the proppant in suspension relatively quickly and then retains its desired range of viscosity for at least several hours at ambient temperatures. At the same time, the gel should, when injected into downhole formations at temperatures of 200.degree.-250.degree. C., retain sufficient viscosity to keep the proppant in suspension for the period of time required for the fracturing operation, which may be up to eight hours.
Even more preferable would be a gelling agent and activator which can be mixed with a liquid hydrocarbon "on the fly", i.e., as the constituents are being pumped into the well bore, wherein the hydrocarbon is gelled almost immediately. Such a gelling agent and activator would eliminate premixing and storage of the gel in a frac tank prior to use, as well as significantly reduce the need to maintain the viscosity of the gel at ambient temperature for any prolonged period. The present inventor undertook the task of developing just such a gelling agent capable of "on the fly" gelling of hydrocarbons.
Historically, the art of gelling organic liquids, e.g., hydrocarbons, for oil field fracturing purposes has involved introducing a phosphate ester and an aluminum source into the organic liquid. The in situ reaction between the phosphate ester and the aluminum source forms an aluminum phosphate ester which, in turn, gels the hydrocarbon.
Early phosphate esters were made by reacting a mixture of alcohols, such as ethyl, octyl, and decyl alcohols, with P.sub.2 O.sub.5 (phosphorous pentoxide). The resulting product was a mixture of the corresponding mono- and di- phosphate esters. These and similar phosphate esters were termed "gelling agent" by the oil field industry.
Various conventional gelling agents are set forth in the following patents and application: U.S. Pat. No. 4,007,128 (Poklacki), issued Feb. 8, 1977; U.S. Pat. No. 3,575,859 (Monroe), issued Apr. 20, 1971; U.S. Pat. No. 4,877,894 (Huddleston), issued Oct. 31, 1989; U.S. Pat. No. 4,200,539 (Burnham et al.), issued Apr. 29, 1980; U.S. Pat. No. 3,494,949 (Monroe), issued Feb. 10, 1970; U.S. Pat. No. 4,104,173 (Gay et al.), issued Aug. 1, 1978; U.S. Pat. No. 3,757,864 (Crawford et al.), issued Sept. 11, 1973; and European Patent Application Publication No. 0 225 661, published Jun. 16, 1987.
U.S. Pat. Nos. 4,007,128, 3,575,859, 3,494,949, 4,104,173, 4,200,539 and 3,757,864 all disclose various gelling agents which include salts of alkyl orthophosphates. In particular, U.S. Pat. No. 4,007,128 provides a partial salt of aluminum alkyl orthophosphates (gelling agent) and a neutralizing amount of polyamine (activator) which retain gel character in the presence of epoxy resins and aromatic diamines. The gelling agent according to this patent is formed from the reaction product of (A) an alkyl acid orthophosphate which is produced from alcohols and phosphorous pentoxide, and (B) a basic aluminum compound, e.g., hydrated alumina or aluminum isopropoxide.
U.S. Pat. No. 3,575,859 discloses the gelling of hydrocarbons with small amounts of metal alkyl oleyl orthophosphates (gelling agent) and amines (activator). The gelling agent is formed from the reaction product of (A) ethyl oleyl orthophosphoric mono acid, i.e., a dialkyl phosphate mono acid, and (B) a metal base, e.g., aluminum, gallium or lanthanum hydroxide. Oleyl is a C.sub.18 alkenyl radical.
U.S. Pat. No. 3,494,949 relates to the improvement of the viscosity of motor oils via the addition of aluminum salts of alkyl orthophosphates. A precipitate of aluminum ethyl oleyl orthophosphate is formed from the reaction product of (A) ethyl oleyl orthophosphate with water and ethanol, (B) sodium hydroxide, and (C) aluminum sulfate.
U.S. Pat. No. 4,104,173 is directed to a gelling agent of pseudo double salts of alkyl orthophosphates. These gelling agents are formed from the reaction product of (A) a mixture of alkanol and/or alkenols and phosphorous pentoxide, (B) a first base, i.e., sodium aluminate or hydrated aluminum oxide, and (C) a second base, i.e., sodium hydroxide.
U.S. Pat. No. 4,200,539 discloses a gelling agent formed from the reaction product of (A) phosphorous pentoxide and an aliphatic alcohol, and (B) an aluminum compound selected from the group consisting of alkali metal aluminate, aluminum isopropoxide, and aluminum hydroxide.
U.S. Pat. No. 3,757,864 discloses a gelling agent of aluminum salts of aliphatic orthophosphate ester formed from the reaction product of (A) orthophosphoric acid ester, (B) phosphorous pentoxide, and (C) a basic aluminum compound, e.g., sodium aluminate, aluminum isopropoxide or hydrated alumina.
European Patent Publication No. 0 225 661 is directed to raising the viscosity of a hydrocarbon by the addition of a phosphate ester and metallic salts which are substantially completely free of water and pH affecting substances. The phosphate ester gelling agent is formed from the reaction product of (A) a phosphate triester and phosphorous pentoxide, and (B) an alcohol. The metal salt activator is typically metal alkoxide or aluminum isopropoxide.
Well operators have been seeking a gelling agent product that is easily pumpable, could be diluted if necessary, and would gel rapidly "on the fly". The aforementioned conventional gelling agents are unable to meet those requirements. Aluminum complexes or salts from aluminum isopropoxide or alumina are not satisfactory because they result in a precipitate, not a liquid product that could gel "on the fly".
The present inventor has discovered through extensive experimentation that the reaction product of selected dialkyl phosphates, an aluminum sulfate and a solvent results in a liquid gelling agent which may be mixed "on the fly" with hydrocarbon and an appropriate activator at or near the wellhead such that a suitable gel forms almost immediately. This unique gelling agent overcomes the disadvantages of conventional gelling agents, i.e., premixing of the fracturing fluid and storage of the gelled fluid in frac tanks.
The gelling agents of the present invention also provide the following economic benefits: (1) no residual gelled fracturing fluid, (2) no disposal of residual gelled fracturing fluid, (3) requires smaller dosages than conventional gelling agents resulting in reduced pumping friction, and (4) power consumption is substantially reduced by the lack of a premixing step and since pumping of gelled fracturing fluid from a frac tank to the wellhead is avoided.
The present invention therefore overcomes the abovementioned deficiencies, as well as provides additional advantages which shall become apparent as described below.